1. Field of the Invention
The present invention relates generally to communication systems that transmit, receive and process communication signals and, more particularly, to providing user specific downlink beamforming in a fixed beam network.
2. Description of the Related Art
This section is intended to introduce the reader to various aspects of art that may be related to various aspects of the present invention, which are described and/or claimed below. This discussion is believed to be helpful in providing the reader with background information to facilitate a better understanding of the various aspects of the present invention. Accordingly, it should be understood that these statements are to be read in this light, and not as admissions of prior art.
Communication systems that transmit and receive communication signals continue to grow in importance. Such systems are used to provide television, radio, satellite communication, cell phone service, wireless computing networks and the like. An important aspect of such systems is the ability to efficiently process signals to continue to improve the quality of service provided to users.
Antenna arrays may be used to perform beamforming to enhance reception of signals from different angles of arrival, and transmit beamforming to enhance the quality of transmission of signals to different users. Phase offsets between signals received from a user on different elements of the antenna array depend on the angle of arrival of the user's signals at the antenna array. This phenomenon can be utilized to combine signals arriving from a desired direction constructively at the base station receiver using a receive beamformer. A receive beamformer is a device that receives inputs from the various elements of an antenna array and combines them into output signals or beams based on certain criteria.
In addition, transmit beamformers may be used to enhance signals prior to their transmission by an antenna array. Transmit beamformers may apply weighting coefficients to the signal intended for any user before transmission by an antenna array such that the desired signal strength for the user is enhanced and/or that the interference caused by this user's signal to other users is reduced. The weighting coefficients applied by a transmit beamformer may be adjusted according to various measurements of the signals received from the desired user at the antenna array or any other knowledge of the user's angle of location from the antenna array. Using transmit beamforming weight coefficients, the signal intended for a desired user may be thought of as being “steered” toward the direction of the desired user, such that the signals strength for the desired user is maximized and interference caused by this signal to users located at other angles is reduced.
Mobile transceivers, such as cellular telephone handsets, may be referred to as user equipment (UE). Channel estimation by a mobile transceiver is very important to realize beamforming gains. In many wireless communication systems, including third generation (3G) systems such as Universal Mobile Telephone Service (UMTS), phase reference signals may be provided to assist mobile transceivers in performing channel estimation and synchronizing on a received signal. The phase reference to be used by a mobile transceiver for a given communication session is typically specified by Radio Resource Control (RRC, or upper layer protocol) signaling. In UMTS and other systems, available phase references may include common pilot channels such as the primary common pilot channel (P-CPICH) and the secondary common pilot channel (S-CPICH). Another pilot channel, which may be referred to as a dedicated pilot channel (DPILOT) may be provided as well.
Because the mobile transceiver typically has no awareness that any type of beamforming is being applied (because of the proprietary nature of base station antenna configurations), the type of phase-reference that the mobile uses for its channel estimation and signal demodulation is an important aspect of the performance of the beamforming algorithm. Thus, beamformers have to be designed keeping in mind the phase-reference that a mobile transceiver is going to use.
Downlink beamforming is a method of signal transmission from a group of closely spaced antennas, such as a cellular telephone base station. Transmission from the base station may be designed such that the signals transmitted to a mobile transceiver all arrive co-phased at the mobile antenna. Because of the closely spaced nature of the base station antennas, the wireless channels from the base station antennas to the mobile antenna are all highly correlated. This correlation is represented by a spatial correlation matrix, which can be measured from uplink pilot signals. The spatial channel correlation is exploited by a beamformer, which applies appropriate complex weights to the signal at the different antenna elements. The weights may be designed such that a particular user's signals from all the transmitting antennas, after going through the channels, arrive coherently (or co-phased) at the user's receiving antenna. This typically results in a signal-to-interference ratio improvement of about a factor equal to the number of transmit antennas. As such, the design of a beamformer is an important element with respect to the performance of any multi-antenna system.
There are two types of beamformers that may be employed in a multi-antenna wireless system. One is the user-specific beamformer, which forms beams on a per user basis, one beam per user. This requires information on the user's channel characteristics, typically obtained through the spatial channel correlation matrix, which is computed based on an uplink pilot received when the mobile transceiver sends data to the base station. This correlation matrix essentially gives a measure of the direction in which the UE is located, which may allow a beamformer to form beams that point in that direction.
User specific beam forming has several shortcomings, however. User specific beamforming may employ the P-CPICH for channel estimation and synchronization purposes, but the P-CPICH may only be effective as a phase reference under certain conditions, such as when employed in systems with very few closely spaced antennas.
However, user specific beamforming strategies that are appropriate for systems that employ a very small number of closely spaced antennas may not work well when extended to systems that have many closely spaced antennas. This is because the correlation across the antennas decreases as the channel becomes more spatially scattered (leading to what is called as high angular spread). The problem of high angular spread arises when there are a large number of spatially dispersed local scatterers around the mobile user. Beamforming becomes less effective when angular spread is high because the signal energy arrives only partially co-phased at the mobile antenna. Further, the fact that the P-CPICH phase-reference is not beamformed leads to much steeper degradation in performance because the pilot and traffic see different channels as angular spread increases. This is known as pilot-to-traffic mismatch and beamforming systems must perform within extremely strict tolerances to optimally realize beamforming benefits in the face of such mismatches.
The other typical type of beamforming system is known as a fixed beamforming system. In fixed beamforming systems, the base station does not form beams appropriate for each and every user, but rather forms a set of few common beams pointing in different predetermined directions, such that the whole cell area of interest is covered. These common beams are called fixed because they do not adapt to any particular user's location. However, these common beams can be made to change from time to time depending on various factors, such as changes in traffic load pattern and the like. As long as the beams are not designed to serve any one particular user, but rather meant to serve a common cellular sub-area, that type of system is referred to a fixed beamforming network.
Fixed beamforming does not suffer from the pilot-to-traffic mismatch problem because the common pilot may be a secondary common pilot channel (S-CPICH), which may be sent over the same fixed beam that is used to serve a user. A user who happens to be at the peak of a beam being used for signal transmission may see the maximum possible beamforming benefit. A steep decline in performance because of steep roll-off of the beam patterns may be experienced by users that are between two beams. This performance decline may be on the order of around three (3) dB for a four (4) antenna base-station.
Fixed beamforming, therefore, is intrinsically not fair, because users get different quality of service (QoS) depending on their geographic location. This situation is clearly undesirable. These “coverage gaps” can be minimized by defining more fixed beams, but defining more fixed beams entails an increased power allocation for the overhead channels. This is true because correspondingly more secondary common pilot channels S-CPICHs would be needed. Another option is to sweep the beams periodically in time. However, this strategy trades the performance losses among the different users in time and does not alleviate the problem of performance loss completely.
Further, if a user moves from the coverage area of one beam to another, there is a typical delay in signaling the user to change its phase-reference (S-CPICH ID) because higher-layer signaling is involved. Because of this delay, the user could continue to use an “old” phase reference for some time, resulting in a much greater degradation of performance. This is an important problem since the beams have a very sharp decline in gain in areas beyond their main coverage areas.